The present invention relates to cylinder fabrication methods, more particularly to methods for fabricating composite cylinders which are suitably used as underwater pressure hulls.
Conventional techniques for fabrication of thick section composite cylinders, i.e., cylinders with relatively high cylinder wall-thickness-to-diameter ratios, have been fraught with problems pertaining to residual stresses. Thick section composite cylinder residual stresses typically load the hoop or circumferential fibers in compression on the outside diameter and tension on the inside diameter. See Lee, Soo-Yong, and G. S. Springer, "Filament Winding Composite Cylinders: II. Thick Cylinders," Stanford University, Department of Aeronautics and Astronautics, Stanford, California, August 1989, incorporated herein by reference, esp. pages 107 and 109. This weakens the cylinder when loaded by external pressure or internal pressure (where the cylinder's strength is based on material properties and not on structural stability) because of this initial compressive stress or tensile stress, respectively. Hence, reducing the residual stresses can help to increase the cylinder's strength.
Strains as high as 20% of the ultimate strains caused by residual stresses have been measured on thick section rings. See Welch, D. E., H. W. Blake, and R. E. Garvey, "Compression Tests of Thick-Composite Rings," Fourth Annual Thick Composites in Compression Workshop, Jun. 26, 1990, incorporated herein by reference, esp. pages 486-487. It has been reported that an eight inch diameter thick section cylinder delaminated while in storage, most likely due to residual stresses. See Frame, B. J., "Failure Analysis of ETAC Pressure Vessel," prepared by the Oak Ridge Gaseous Diffusion Plant for the U.S. Department of Energy under Contract DE-ACO5-840R21400, June 1987, incorporated herein by reference, esp. pages 13-15; in this reported case, the residual stresses apparently developed strains of 100% of the ultimate in the cylinder wall.
There is not an abundance of published information on composite pressure hull fabrication techniques, as only a limited number of scale hull cylinders have been built. Many of these were filment wound monolithic sections. See Stachiw, J. D., and B. Frame, "Graphite-Fiber-Reinforced Plastic Pressure Hull Mod 2 for the Advanced Unmanned Search System Vehicle,"Technical Report 1245, Naval Ocean Systems Center, San Diego, August 1988, incorporated herein by reference, esp. pages 16-20, 41, 44 and 83. Some of these monolithic cylinders had ring stiffeners machined into the inner diameter. See Rasmussen, E. A., and J. R. Carlberg, "Shock Response of Ring Stiffened Glass/Epoxy," David Taylor Research Center, U.S. Navy, DTRC-SSPD-91-172-75, Bethesda, Maryland, April 1991, incorporated herein by reference, esp. pages 1, 7, 43 and 46. Other cylinders had ring stiffeners bonded to the inner diameter. See Harruff, P. W., Frank C. Spicola, and T. Tsuchiyama, "Filament Wound Torpedo Hull Structures," Society of Manufacturing Engineers, 1986, Dearborn, Michigan, incorporated herein by reference, esp. pages 12-16 and 18.
The U.S. Navy has devoted considerable effort toward development of composite underwater pressure hulls for its submarines and other submersible vehicle structures. Small scale hulls have typically been thick-walled filament-wound cylinders with, if required, circumferential ribs machined out from the inside.
Utilization of a similar fabrication process for large scale hulls would be expensive; it would require a large investment in capital equipment, advances in thick section composite quality control and advances in state of the art "non destructive examination" (NDE).
Although other proposed fabrication methods can reduce fabrication equipment requirements, they still produce a monocoque hull, thus posing the same NDE difficulties. Few, if any, of the fabrication processes developed and proposed for circumferentially stiffened pressure hulls allow the integral fabrication of the stiffeners, have the stiffeners as an integral part of the hull, or provide for radial reinforcement of the stiffeners.